1. Field of the Invention
The present invention relates to an optical communication system and, more particularly to an optical transmitter for generating optical signals.
2. Description of the Related Art
Optical communication systems utilizing a Dense-Wavelength-Division-Multiplexing (DWDM) scheme have an excellent communication efficiency as they permit the transmission optical signals of multiple channels with different wavelengths through a single optical fiber. The DWDM systems can also transmit optical signals having different transmission speeds. As such, the DWDM are now widely used in ultra-high speed Internet networks, and systems capable of transmitting more than a hundred channels through a single optical fiber, using the DWDM technology, are in common use. Various research efforts have been focused recently to develop a system that is capable of simultaneous transmission of more than two hundred channels of 40 Gbps through a single optical fiber at a transmission speed of more than 10 Tbps.
In order to cope with a rapidly-increased demand in data traffic, there has been much improvement in the wavelength division multiplexing technology. Today, the bandwidth of an Erbium-Doped-Fiber-Amplifier (EDFA) is approaching its limit which is based on the intensity modulation technology used commonly in the art. Therefore, a new technology other than EDFA is needed in the near future to meet the steady increase in data traffic. As an approach to increase the transmission capacity of the DWDM system, “duobinary” transmission technology has emerged to improve the bit spectral efficiency by utilizing the limited bands of the optical fibers efficiently.
FIG. 1 shows a configuration of a conventional duobinary optical transmitter. The transmitter comprises first and second low-pass filters 120 and 150; first and second modulator driver/amplifier 130 and 160; a laser source 170; and, a Mach-Zehnder interferometer-type optical intensity modulator 180.
In operation, a two-level binary data signal is inputted to the first low-pass filter 120. The first low-pass filter 120 has a bandwidth corresponding to about ¼ of the clock frequency of the two-level binary signal. Due to an excessive limitation on the bandwidth, an interference among codes is generated. This causes a conversion of the two-level binary signal into a three-level data signal. As such, the three-level signal is inputted to the first modulator driver 130. Then, the first modulator driver 130 amplifies the three-level signal and outputs it to the optical intensity modulator 180. The first modulator driver 130 and the optical intensity modulator 180 both have a bandwidth corresponding to a clock frequency of the two-level binary signal. Similarly, a binary inverted data signal is also converted into a three-level inverted data signal through the second low-pass filter 150 during the operation. This three-level inverted signal is amplified through the second modulator driver 160 and then inputted to the optical intensity modulator 180. Lastly, the laser source 170 outputs light with a predetermined wavelength, and the optical intensity modulator 180 modulates the light according to the three-level signals and outputs them for a subsequent transmission.
Note that the first low-pass filter 120 and the first modulator driver 130 constitute a first arm 110 relative to the optical intensity modulator 180, and the second low-pass filter 150 and the second modulator driver 160 constitute a second arm 140 relative to the optical intensity modulator 180. The first and second arms 110 and 140 are connected to the first and second modulation terminals (RF+ and RF−) of the optical intensity modulator 180, respectively.
However, in the duobinary optical transmitter as shown in FIG. 1, only a low-pass filter has a bandwidth corresponding to ¼ of the clock frequency of the two-level binary signal. The remaining modulator driver and optical intensity modulator both have a bandwidth corresponding to a clock frequency of the two-level binary signal and tend to be expensive to implement. Accordingly, the conventional transmitter has a drawback in that using devices of high bandwidth increases the manufacturing cost of the entire optical transmitter, and therefore the cost competitiveness of the optical transmitter is weakened.
Therefore, there is a need for an optical transmitter that may be realized in a simple, reliable, and inexpensive implementation.